a) Field of the Invention
This invention relates to a hybrid vehicle (HV) having a motor and an internal combustion engine (ICE), and particularly to a controller for the HV.
b) Description of the Related Art
In order to improve the fuel consumption of vehicles and to prevent air pollution, various types of HVs having an ICE and a motor have been developed. Among such vehicles, the HV disclosed in Japanese Patent Laid-Open Publication No. Hei 6-80048 has a controller for selectively performing the following driving modes: a sole-motor mode for driving the vehicle by a motor alone, a sole-ICE mode for driving by an ICE alone, and a motor/ICE mode for driving by both the motor and the ICE. A controller mounted on the HV determines according to the HV's driving conditions which mode should be used to drive the HV, and controls respective components such as the above-mentioned ICE and motor to implement the determined mode.
The "driving condition" used here is information on the vehicle speed, the accelerator angle, and the like. For example, the sole-motor mode and sole-ICE mode are basically used when the vehicle speed is low or when the vehicle speed is high, respectively. The motor/ICE mode is used when the accelerator angle is large, i.e., when the vehicle driver requests rapid acceleration, regardless of the vehicle speed (see FIG. 3 of the above-mentioned publication, in which the vehicle speed is shown on the horizontal axis and the accelerator angle on the vertical axis).
In the above-mentioned publication, the driving mode is switched to another driving mode when the index representative of the driving condition falls down below or exceeds a threshold value which is variable according to a state of charge (SOC) or a depth of discharge (DOD) of the on-board battery for supplying the motor with driving electric power and a temperature of the motor (See the arrows in FIG. 3 of the above-mentioned publication and also FIG. 4 of the same publication in which the horizontal axis shows the motor temperatures and the vertical axis shows the threshold value of the accelerator angle.).
In the above prior art a driving mode is determined and switched according to the driving conditions detected prior to the determination. However, such a sequence leads to the controlling delaying the output torque to be supplied to driving wheels. For example, when the accelerator pedal is depressed sharply when the vehicle is running at a low speed, the controller determines that it is necessary to switch the driving mode from the sole-motor mode to the motor/ICE mode, and then issues a command to start the ICE. Here, in general, some start-up period is required for the ICE. Therefore, when switching the driving mode from the sole-motor mode to the motor/ICE mode, after the determination of mode switching is made and the command to perform the mode switching is issued, the driver must wait for the start-up of the ICE.